Vulcanization of rubber



Reissued Apr. 13, 1943 UNITED STATES PATENT OFFICE VULCANIZATION OFRUBBER Paul C. Jones, Silver Lake, and Roger A. Mathes, Akron, Ohio,assignors to The B. F. Goodrich Company, New York, N. Y., a corporationof New York No Drawing. Original No. 2,288,194, dated June 30, 1942,Serial No. 255,358, February 8, 1939. Application for reissue November6, 1942, Serial 21 Claims.

aliphatic or aromatic groups may be employed in connection with themonocarboxylic acids or their metallic salts to accelerate thevulcanization of rubber. Such compounds as 2-mercapto4-methylthiazoline; 2-mercapto 5-methylthiazoline; Z-mercapto4-phenylthiazoline; Z-mercapto 4-methoxymethylthiazoline; 2-mercapto4-anisylthiazoline; 2-mercapto 4chlormethylthiazoline; 2-mercapto4-(p-amino) phenylthiazoline; Z-mercapto 4,4-diethylthiazoline; 2-mercapto 4,5 dimethylthiazoline; 2 mercapto 4,5tetramethylenethiazoline; and similar compounds are within the scope ofthis invention.

It has also been found that the metallic salts function in the samemanner as the thiazolines themselves. This was to be expected, forduring vulcanization the metallic salt could break down to form themercaptothiazoline, or the mercaptothlazoline could react with anymetallic groups in the composition to form the salt. Any metallic saltssuch as the sodium, potassium, calcium, barium, magnesium, tin, lead,zinc, mercury, cadmium and other alkali, alkaline earth and heavy metalsalts of 2-mercaptothiazoline or its substitution products in which oneor more of the hydrogens on the carbon atoms are replaced by aliphaticor aromatic groups may be employed. In its broadest scope, accordingly,the invention consists in vulcanizing rubber in the presence of amonocarboxylic acid or a metallic salt thereof and a compound containingthe structure wherein R represents hydrogen or a combined metal.

As an illustration of the effectiveness of 2-mercaptothiazoline whenemployed in accordance with the method of this invention, the followingcompositions were prepared:

A B C Composition H costar- 0.7018

c OOODCHWOIO -1 mascotm R ubber 1 2-mercaptothi Stearic acid Laurie acidZinc stearate Benzoic acid ocowwwm oowowpmo cwoowpmo After the rubberstocks thus compounded were cured by heating in a mold for varying timesand temperatures, the vulcanized compositions had the followingproperties, T signifying tensile strength at break in lbs/in. and Esignifying the ultimate elongation in percent.

Composition Cure time Temp.

inm'in. deg.F. A B C '1 E 'r E T E D E F '1 E T E 'r E It will beobserved that excellent tensile strengths were obtained by using theaccelerators of this invention, and that tight cures were obtained as isshown by the relatively low elongations of the well-cured stocks. Inspite of the great activity exhibited by the mercaptothiazoomposition AB C When these compositions were cured in a heated mold for varyingtimes at 287 F., compositions having the following characteristics wereobtained, T signifying tensile strength in lbs/in. and E signifying theultimate elongation in percent.

Composition Time of cure in min.

at 287 F. A B i C T E 'r E T i:

It will be observed that while the metallic salt of themercaptothiazoline was a poor accelerator when used alone, it possessedgreat activity when used in the presence of a monocarboxylic acid or ametallic salt of a monocarboxylic acid.

In addition to showing great activity at vulcanizing temperatures,certain members of this new class of accelerators, especially the zincsalts of the Z-mercaptothiazolines, exhibit excellent delayed action.Thus, if 2-mercaptothiazoline is used as the accelerator in CompositionB, it forms a rather scorchy compound which attains a tensile strengthof about 1,7001bs./in. in 30 minutes at 220 F. This makes the processingof the stock rather difficult. On the other hand, Composition Bcontaining the zinc salt of Z-mcrcaptothiazoline does not cure at all in30 minutes at 220 F. and only reaches a tensile strength of about 1,300lbs/in. in 60 minutes at 220 F. This inactivity at processingtemperature does not extend into the curing range, however, as can beseen by the curing characteristics of composition B at 287 F.

As further embodiments of the method of this invention, differentaccelerators were included in the following base recipe:

When the compositions were vulcanized, products having the followingcharacteristics were obtained:

Salt of Z-mercaptothiazoline used Any of the other metallic salts ofZ-mercaptothiazolines may likewise be used as accelerators in accordancewith the method of this invention. The alkali metal salts are valuablecompounds in that they are soluble in water, and may accordingly be usedto advantage in compounding latex, it being possible to keep the latexindefinitely without the accelerator precipitating therefrom.

It will be observed that many of the compositions within the scope ofthis invention contain zinc oxide, a Z-mercaptothiazoline, and a fattyacid. It has been found that these compounds may be reacted beforeincorporation in the rubber to form mixed zinc salts of fatty acids and2- mercaptothiazolines containing the structure wherein A is a fattyacid group. This modification of the invention is advantageous becausethe relatively high-melting zinc salts of 2-mercaptothiazolines aresometimes rather difiicult to disperse, while the lower-melting mixedzinc salts of a 2-mercaptothiazo1ine and a saturated or unsaturatedfatty acid, preferably one containing eight or more carbon atoms, haveequivalent accelerating activity and are easily dispersed in rubher.

The mixed zinc salts of 2-n1ercaptothiazolines and fatty acids may beprepared by reacting a fatty acid, zinc oxide and a2-mercaptothi'azoline in a 1 l 1 ratio, one mole of water being splitoff. Thus, 8.1 parts by weight of zinc oxide, 28.4 parts of stearicacid, and 11.9 parts of 2-rnercaptothiazoline are reacted by stirringand heating the melted reactants until 1.8 parts. of water are given offand a homogeneous mixture is obtained. The mixed salts are in generalsoaplike materials which melt below 120 C., and are much more easilydispersed in rubber than the higher-melting zince salts ofmercaptothiazolines. The zinc salt of 2-mercaptothiazoline, forinstance, from which the foregoing accelerator was prepared melted atabout 235 C., while the mixed salt melted at or about -120 C.

To demonstrate the effectiveness of the accelerators of this, invention,the following compositions were prepared:

Composition Min. of cure C It will be observed that all of the materialstested were good accelerators of vulcanization.

Any of the saturated or unsaturated monocarboxylic acids such ascaprylic, palmitic, oleic, ricinoleic, benzoic, salicylic, toluic, orother like acids or their salts with metals such as lead, tin, sodium,cobalt, nickel, calcium, magnesium, and other alkali, alkaline earth,and heavy metals may be employed in place of the materials used in thespecific examples. While the lower fatty acids may be employed ifdesired, it is preferable to employ their metallic salts, zinc acetatefor instance, which are solids. The acids or their metallic salts may bepresent in any desired small proportions, even minute quantities usuallyhaving a beneficial effect. For the best results however, they should beincluded in the compositions in amounts varying from 1 to 5% or more.

It is a remarkable and unusual property of the materials of thisinvention that they are activated by such materials as benzoic acid andsalicylic acid which are widely used retarders for other accelerators.Referring to the table above, it will be seen that at the two cures atwhich compositions containing the material were tested, benzoic acid wasa better activator than any of the other acids or salts employed. Thisunexpected result indicates that mercaptothiazolines possess propertiesnot shared by other accelerators of vulcanization.

Although the use of the accelerators of this invention has beendescribed in detail in connection with specific rubber compositions, itwill be evident that this invention is applicable to rubber compositionsof the :most varied nature, and that all manner of rubber goodsincluding pneumatic and solid rubber tires, tubes, hose, belting,packing, boots and shoes, surgical rubber goods, seamless dipped rubberarticles, etc. may be vulcanized in the presence of the acceleratorsherein described. The accelerator may be incorporated in the rubber bymastication, milling or any similar process, or in the case of latex,rubber cement, or any other natural or artificial dispersion or solutionof rubber, by simply dissolving or suspending the accelerator therein.

The vulcanization may be performed in other manners than thatparticularly set forth in the specific examples; specifically, rubbercompositions may be heated in the presence of sulfur or selenium in hotair, steam, hot water, etc.

It is to be understood that the term a rubber is employed in theappended claims in a generic sense to designate those rubbery materialswhich are vulcanizable with sulfur, including caoutchouc, balata, guttapercha, latex, rubber isomers, or synthetic rubber whether or notadmixed with pigments, fillers, softeners, antioxidants, otheraccelerators, etc., but not including materials incapable ofvulcanization by reaction with sulfur.

While we have herein disclosed specific em bodiments of our invention,we do not intend to limit ourselves solely thereto, for it will beobvious to those skilled in the art that many modifications such asusing other materials having equivalent properties and varying theproportions of materials used are within the spirit and scope of theinvention as defined in the appended claims.

We claim:

1. The process which comprises vulcanizing a rubber in the presence of amember of the class consisting of 2-mercaptothiazolines and saltsthereof in which a metal is combined solely with the acid radicalsthereof, and a member of the class consisting of monocarboxylic acidsand their metallic salts.

2. The process which comprises vulcanizing a rubber in the presence of aZ-mercaptothiazoline and a member of the class consisting ofmonocarboxylic acids and their metallic salts.

3. The process which comprises vulcanizing a rubber in the presence ofZ-mercaptothiazoline and a monocarboxylic acid.

4. A ru-bber composition comprising a rubber which has been vulcanizedin the presence of a 2- mercaptothiazoline and a member of the classconsisting of monocarboxylic acids and their metallic salts.

5. A rubber composition comprising a rubber which has been vulcanized inthe presence of 2- mercaptothiazoline and a member of the classconsisting of monocarboxylic acids and their metallic salts.

6. The process which comprises vulcanizing a rubber in the presence of ametallic salt in which the metal is combined solely with the acidradicals of 2-mercaptothiazolines, and a member of the class consistingof monocarboxylic acids and their metallic salts.

7. The process which comprises vulcanizing a rubber in the presence of asalt consisting of zinc combined solely with the acid radicals of2-mercaptothiazolines, and a member of the class consisting ofmonocarboxylic acids and their metallic salts.

8. The process which com-prises vulcanizing a rubber in the presence ofthe zinc salt of 2-mercaptothiazoline and a member of the classconsisting of monocarboxylic acids and their metallic salts.

9. A rubber composition comprising a rubber which has been vulcanized inthe presence of a metallic salt in which the metal is combined solelywith the acid radicals of Z-mercaptothiazolines, and a member of theclass consisting of monocarboxylic acids and their metallic salts.

10. A rubber composition comprising a rubber which has been vulcanizedin the presence of a salt consisting of zinc combined solely with theacid radicals of 2-mercaptothiazolines, and a member of the classconsisting of monocarboxylic acids and their metallic salts.

11. A rubber composition comprising a rubber which has been vulcanizedin the presence of the zinc salt of 2-mercaptothiazoline and a member ofthe class consisting of monocarboxylic acids and their metallic salts.

12. The process which comprises vulcanizing a rubber in the presence ofa Z-mercaptothiazoline and a fatty acid.

13. The process which comprises vulcanizing a rubber in the presence ofa Z-mercaptothiazoline and a. metallic salt of a fatty acid.

14. The process which comprises vulcanizing a rubber'in the presence ofa fatty acid and a metallic salt in which the metal is combined solelywith the acid radicals of 2-mercaptothiazolines.

15. The; process which comprises vulcanizing a rubber in the presence ofa fatty acidand, a salt consisting of zinc combined solely with the acidradicals of Z-mercaptothiazolines.

16. The process which comprises vulcanizing a rubber in the presence of2-mercaptothiazoline and a fatty acid.

1'7. The process which comprises vulcanizing a rubber in the presence 0fv 2imercaptothiazoline and the zinc salt of a fatty acid.

18. The process which comprises vulcanizing a rubber in the presence ofa fatty acid and the zincsal-t of 2nmercaptothiazoline.

19. A rubber composition comprising a rubber which has been vulcanizedin the presence of a fatty acid. and a metallic salt in which, the metalis, combined solely with the acid radicals of 2? mercaptothiazol'ines.

20. A rubber composition comprising a rubber which has been vulcanizedin the presence of a fatty acid and a salt consisting of zinc combinedsolely with the acid radicals of 2-mercaptothiazolines.

21'. A rubber composition comprising a rubber which has been vulcanizedin the presence of a 2- mercaptothiazoline and a fatty acid.

PAUL C. JONES. ROGER A. MAT'HES.

